Example: if you enter $100,000 into the official calculator and the Excel formula, they will match. However, if you enter $100,001, the official calculator will say $2438.50 and the Excel formula will say $2435.04. In fact, if you enter any figure between $100,000 and $199,999, their calculator will give the same response. I'm not sure whether the Tasmanian SRO round their inputs to the nearest $100, or whether they follow the letter of the law when calculating liability.

I've created an additional Excel formula which replicates the behaviour of the online calculator.
You're free to pick which one you like. Either way, the maximum margin of error for this formula will be $4.50 (the liability from the highest range). If an inaccuracy of $4.50 bothers you, perhaps you shouldn't be in the property market!

Standard disclaimer applies: do not trust this formula until you have tested it yourself. It makes all sorts of assumptions like you are an Australian citizen and are not entitled to any concessional rates.